Airplane heater having two-speed control



June 2l, 1949.

c. H. BENNETT AIRPLANE HEATER HAVING TWO-SPEED CONTROL Filed June 20, 1944 /lll June 21, "1949. c. H. BENNETT 2,473,699

AIRPLANE HEATER HAVING TWO-SPEED CONTRL Filed June 20, 1944 2 Sheets-Sheet 2 lNvENToR M ATTORNEY Patented June 21,l 1949 AIRPLAN E HEATER HAVING TWO-SPEED CONT ROL

Clarence H. Bennett, Philadelphia, Pa., assigner to Selas Corporation of America, Philadelphia, Pa., a corporation of Pennsylvania Application June 20, 1944, Serial No. 541,187

2 Claims.

My invention relates to airplane heaters.

It is an object of my invention to provide an improvement for controlling the rate at which air for combustion is supplied to an airplane heater with changes in altitude, whereby the desired weight ratio of air to fuel may be maintained under all operating conditions encountered in aircraft heating.

Another object of the invention is to provide such an improvement for controlling the rate at which air for combustion is supplied to an airplane heater which is especially7 useful for a heater operable at low and high heat outputs.

y A further object of the invention is to provide an improved control system for operating an airplane heater at low and high heat outputs in which a fan for circulating air to be heated and an impeller for supplying air for combustion are operable at two speeds or speed ranges and liquid fuel is supplied to the heater at a different fixed f delivery pressure for each speed or speed range.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the claims. The invention, both as to organization and method, together with further objects and advantages thereof, will be better understood from the following description taken in connection with the accompanying drawings forming a part of this specification, and of which:

Fig. 1 is a vertical sectional view diagrammatically illustrating an airplane heater embodying the invention;

Fig. 2 is a view diagrammatically illustrating an electrical circuit embodying the invention for controlling the heater shown in Fig. 1;

Fig. 3 is a View diagrammatically illustrating a fuel supply system for the heater shown in Fig. 1 with which is associated the electric circuit shown in Fig. 2;

Figs. 4, 5 and 6 are fragmentary views diagrammatically illustrating different operating positions of a part shown in Fig. 2, to illustrate the invention more clearly; and

Fig. 7 is a fragmentary view in elevation illustrating a control panel for the electrical circuit shown in Fig. 2.

Referring to Fig. 1, I have shown my invention in connection with an airplane heater 9 generally as disclosed in application Serial No. 450,577, filed July l1, 1942, of Frederic O. Hess and Carl P. Mann, now Patent No. 2,388,970, granted November 13, 1945. This heater comprises a radiator or heat dissipating element I0 which is annular in form and concentrically disposed within an outer cylindrical casing I I. The radiator I0 includes aA pair of spaced apart metallic walls I2 and I4 having a plurality of heat transfer fins I5 extending into a heating space I6 therebetween, and additional heat transfer fins I'I extending into annular spaces 8 and I9 through which air to be heated is circulated.

A burner 20, which is annular in form and receives the lower edges of the radiator walls I2 and I4, is formed with an inlet chamber 2| which receives a combustible fuel mixture, as will be described presently. The mixture passes from the chamber 2l through a number of small passages 22 into the bottom part of the heating space I6. A number of relatively thin plates 23 of refractory material, having slots or channels in the faces thereof, may be held closely together in abutting relation about the upper part of the annular inlet chamber 2l to provide the passages 22 for subdividing the fuel mixture into a number of small jets or streams.

Ignition of the fuel mixture may be effected by an ignitor 24 which extends through the casing I I and wall I2 into the lower part of heating space I6. By subdividing the fuel mixture into a plurality of jets or streams, a plurality of tiny flames are produced and maintained at the upper ends of the passage 23 after combustion has been started. Substantially complete burning or combustion of the fuel mixture takes place in the region 25 adjacent to and in the immediate vicinity of the passages 23, and it is for this reason that the iins I5 extending into the space I6 are cut away at their lower ends, as indicated at 26.

The high temperature products of combustion pass upwardly within the heating space I6 and give up heat to air which is circulated upwardly through annular spaces I3 and I9 by an air circulating fan or propeller 2l positioned at the outlet 2S. The air to be heated, which is drawn from the surroundings of the heater within the interior of the airplane, passes into the lower ends of the annular spaces I9 and I9 at the inlet 29, and the heated air is discharged at the outlet 28 to which may be connected ducts (not shown) for distributing the heated air to a number of places within the airplane.

The products of combustion pass from the upper end of the heating space I6 into a manifold 30 having a plurality of inwardly and downwardly extending arms 3| which are connected to the upper ends of a number of vertical tubes 32. The lower ends of tubes 32 are connected to an inlet chamber 33 of an impeller housing or blower 34 having a plurality of chambers 35 and 36 separated by a dividing wall 31 and within which are disposed rotatable impellers 38 and 39, respectively. The impellers 38 and 39 are arranged to be driven by an electric motor 40 which is mounted on housing 34 and also drives the air circulating fan 21.

The products of combustion passing into inlet chamber 33 are drawn into the upper impeller chamber 35 by the impeller 38 and discharged is delivered from a suitable source of supply through a conduit 44 to the end of which is secured a small spray nozzle 45 disposed below the inlet of the lower impeller chamber 36. A small stream of liquid fuel, which is discharged upwardly from the nozzle 45 into the lower impeller chamber 36, strikes the rotating impeller 39 and is mechanically atomized and mixes with the combustion air. The mixture of combustion air and fuel, which is in liquid phase but in mist or spray form, passes from chamber 36 through passage 5| into the kburner inlet chamber 2l from which the combustible fuel mixture flows through the passages 22 into the lower end of the heating space I6, as previously described.

Liquid fuel is supplied under pressure to the heater 9 just described .by a pump 46, as shown in Fig. 3. The pump 46 receives fuel from a vessel 41 through a conduit 48 including a filter 49 for removing foreign matter, and delivers fuel under pressure to the nozzle 45 through conduits 50 and 44 in which are connected suitable control devices to be described hereinafter. A suitable conventional bypass conduit 52 having an adjustable pressure relief valve 53 connected therein is provided for returning fuel from pump 46 to vessel 41 when the fuel pressure at the discharge side of the pump exceeds a denite value which will cause opening of valve 53.

In accordance `with my invention, in order to control the varying density to the heater with changes in altitude of an airplane in which the heater may be installed, I provide an improved altitude com pensator 54 in the upper end of an atmospheric air intake manifold 55 which is connected at its lower end to conduit 43. The altitude compensator 54 includes a cylindrical sleeve 5B having an inwardly extending flange 57 at its lower open end which bears against a shoulder 59 formed within the upper wider part of manifold 55. The extreme upper end of manifold 55 threadedly receives a flanged cap 59 which bears against the periphery of the closed end of sleeve 56 to hold the latter securely in position.

An expansible and contractible bellows 30 is supported within sleeve 56 b y a threaded pin 6| which extends through the upper closed end of the sleeve and receives a lock nut E2. To the lower end of lbellows 60 is secured a cup-shaped valve member 63 having a plurality of small openings 54 in the bottom and a number of elongated vertical slots 65 in spaced relation about its side wall. The manifold 55 and sleeve 55 are both provided with slots which are at the same elevation, and, when the sleeve is positioned admission of atmospheric air of Within the manifold so that these slots are in alignment, openings 66 are formed through which atmospheric air for combustion may pass into the interior of the sleeve.

IThe bellows 60 of the altitude compensator is axially adjustable by the pin 6I and lock nut 62 so that the valve member 63 is in the position illustrated in Fig. l when the heater is being operated at sea level. Under these conditions atmospheric air is drawn into the lower impeller chamber 36 by impeller 39 through openings 66. the interior of sleeve 56, the small openings 64 in valve member 63, manifold 55 and conduit 43.

For reasons which will be given presently, the bellows 6D, Valve member 63 and flange 51 forming the valve seat are positioned with respect to each other so that atmospheric air for combustion can only pass through openings 64 from sea level to a predetermined high altitude. In other words, as the atmospheric pressure decreases with increase in `altitude from sea level to the predetermined high altitude and the bellows 60 expands with su-ch decrease in atmospheric pressure, the downward movement of valve member 63 is insuineient to permit flow of atmospheric air from the interior of sleeve 56 into the upper part of manifold 55 through the slots 65 and air can only pass through the small openings 64.. However, at altitudes above the predetermined high altitude, the expansion of bellows 60 with decrease in atmospheric pressure is such that valve member B3 will move downward suiciently past valve seat 5l, so that atmospheric air will not only pass through the small openings 64 into manifold 55 but also through slots 65 which now extend below the bottom edge of the valve seat. Under these conditions larger volumes of atmospheric air are supplied for combustion since air can pass through both the slots 65 and the openings 64 into the upper part of manifold .55 and thence into the impeller chamber 36. As the altitude increases from the predetermined altitude the bellows 69 continues to expand until the slots 65 are wide open at a definite higher altitude.

In order to promote stable combustion conditions and reliable heater operation when the heater 9 is being operated either at sea level or at different altitudes as high as 30,000 feet or higher, it is desirable to maintain a proper weight ratio of air to fuel under all of these operating conditions encountered in aircraft heating. Since liquid fuel is substantially unaffected by change in atmospheric pressure and may be supplied to the heater at a substantially uniform rate from sea level to high altitudes, it is necessary to supply larger volumes of air at increasingly higher altitudes to maintain a proper weight ratio of air to fuel as the air density decreases.

In some instances it is possible to operate the airplane heater 9 satisfactorily from sea level to a predetermined high altitude by supplying all of the air for combustion through the small openings 64 in valve member 63, Aas just pointed out. This .is particularly true when the electric motor 4!) driving the combustion air impeller 39 is -a series wound motor, because a motor of this type can drive the combustion air impeller at an increasingly higher speed with increase in altitude so 'that larger volumes of combustion air will be supplied to the heater as the air density decreases. Since the resistance to flow of atmospheric air in the supply duct formed by conduit 43 and manifold 55 decreases with decrease in air density, this also lcontributes to enable larger volumes of combustion air to be supplied with increase in altitude of the airplane. Further, the velocity at which atmospheric air flows through the openings 64 increases with increase in altitude, because the rate of flow of a gaseous fluid through an orifice increases with decrease in density of the fluid when other conditions, such as temperature, for example, remain the same.

Inasmuch as any heater like that described is intended, when operated alone or in conjunction with one or more other heaters, to maintain the interior of an airplane comfortable at a temperature of about 70 F. during normal operation, and air for combustion is withdrawn from the surroundings of the heater within the interior of the airplane, the temperature of the atmospheric air utilized for combustion for all practical purposes will always be substantially the same when operated at sea level or at altitudes up to 30,000 feet or higher.

Hence, when the combustion air impeller 39 is driven by a series type motor and consideration is given to the rate at which the speed of such a motor increases with increase in altitude, and to the decrease in resistance to ilow i air in the supply duct and to the increase in velocity of air passing through the openings 64 due to decrease in air density with increase in altitude, it is possible to maintain a weight ratio of air to fuel which is substantially constant for all practical purposes from sea level to a predetermined altitude even when all of the air for combustion is supplied only through the openings 64.

It has been found that the weight ratio of air to fuel can be maintained substantially constant for all practical purposes to a predetermined high altitude, such as 15,000 feet, for example, when air for combustion is only permitted to flow through the small openings G4. At altitudes above such predetermined altitude, the rate of increase in motor speed with additional increase in altitude ordinarily is not suliicient to maintain the desired weight ratio of air to fuel when the field excitation remains the same, and it is at this time that the bellows Si? expands and elongates sufficiently to cause the slots 85 to extend beyond the lower edge of the valve seat l and permit additional air for combustion to be supplied to the heater 9. The rate at which the slots 65 open with increase in altitude above the predetermined altitude is such that the desired weight ratio of air to fuel may be maintained to altitudes of 30,000 feet and higher.

The combustion air impeller 39 in practice is selected so that atmospheric air for combustion will be supplied at an adequate rate to maintain the desired weight ratio of air to fuel at an altitude of about 30,000 feet or higher. To maintain this desired weight ratio of air to fuel for sea level operation of the heater, the combustion air impelier has to be choked so that the same weight of air will be supplied in a given length of time as at high altitudes. This choking of the combustion air impeller 39 is accomplished by the altitude compensator 54 which only permits air for combustion to pass through the small openings 64 for sea level operation, and the rate at which atmospheric air is supplied at such time is dependent upon the number and size of the small openings 64 formed in the valve member G3.

An airplane heater having an altitude compensator 54 like that just described is especialiy useful for an airplane heater capable of being operated at two different heat outputs, and this may be accomplished in heater 9 by operating the motor 40, at two speeds or speed ranges and supply- 6 ing the liquid fuel at a diierent flxed delivery pressure for each speed or speed range. Such an arrangement for controlling the operation of heater 9 is illustrated in Figs. 2 and 3 whereby the heater may be operated at low and high heat outputs by operating the motor 40 in two speed ranges in each of which the fuelis delivered by the pump 46 at a diiferent fixed delivery pressure to the heater. l

This is accomplished by providing two pressure regulating valves 61 and 68 in the parts of conduits 44 and 5D which are connected in parallel, as shown in Fig. 3. Solenoid operated valves 69 and 'l0 are connected in conduits 44 and 54 at the discharge sides of the pressure regulator valves 61 and G8, respectively, and operate be-.

tween closed and full open positions either to shut off or permit ow of fuel to the nozzle 45. The pressure regulator Valve 68 is adjusted so that fuel may be supplied from pump 45 to nozzle 45 at a low fixed delivery pressure when solenoid operated valve 10 is energized and in its open position and solenoid operated valve 69 is deenergized and in its closed position. The pressure regulator valve 61 is adjusted so that fuel may be supplied from pump 46 to nozzle 45 at a higher fixed delivery pressure when both solenoid operated valves 69 and 10 are energized and in their open positions.

The electric motor 48, igniter 24, motor for driving fuel pump 46, solenoid operated valves 69 and 'l0 and several thermal switches are connected in a single Wire grounded electrical system which is controlled by a single control member 1I, as shown in Fig. 7. The control member 'Il is arranged to operate a four position snapacting rotary switch 'l2 of any well known type which is connected by a conductor 13 to the positive side of a suitable direct current source of supply 'i4 having its negative side grounded, as shown in Fig. 2. The conductor 13 is connected to three switch arms 15, 16 and 'll which are rotated by turning control member ll and arranged to engage one or more of contacts A, B, C, D and E in different positions of the control member 1|, as shown in Figs. 4, 5 and 6.

A thermal switch 98 serves to deenergize the ignitor 24 after combustion has been accomplished in heater 9, and thermal switch 85 opens to shut off the supply of fuel to spray nozzle 45 when undesirable overheating of the heater occurs. Although the thermal switches 85 and 80 are diagrammatically illustrated as being of a bi-metallic type, it is to be understood that these switches may be of any suitable type and may be positioned in annular space I9 against wall I2, as shown in Fig. 1.

When the control member 'H is moved from the Off position to the High position on panel 18 upon which it is mounted, the switch arms are rotated from the Off position in Fig. 3 to the High heat position in Fig. 6, so that switch armsv 15, 16 and 'l1 engage contacts D, E and A, respectively. This completes a circuit for a heating coil of a time delay relay 8l inciuding a bi-metallic disc or strip 82 which is in the immediate vicinity of coil 88 and arranged to close and open contacts 83, 84 and 85, 87. When heating coil 80 is energized, a circuit is completed for electric motor 40 after a definite interval of time from contact A through cooperating contacts 83 and 84 which are closed by movement of the bi-metallic disc 82 due to heating by coil 80. After heating coil 80 has been energized for the definite time interval, a circuit is completed for i? solenoid operated valve 18 from contact D through the normally closed thermal switch 85 and cooperating contacts 86 and 91 which are also closed by movement of bi-metallic disc 82.

From contact D a circuit is also completed through thermal switch 85 for a solenoid operated valve 88 and an electric motor 89 which is arranged to drive fuel pump 46. As shown in Fig. 3, solenoid operated valve 88 is connected in conduit 48 and opens when energized to permit flow of fuel from vessel 41 to pump 46. With the rotary switch 12 in the High heat position in Fig. 6, a circuit is also completed from contact E for solenoid operated valve 69, and a circuit is completed for ignitor 24 from contact D through normally closed switch 90. An electric lamp 9i,

which is connected across thermal lswitch 90, is.

positioned on the panel 18 and becomes lighted when the thermal switch 99 opens.

Upon moving the rotary switch 12 to the High hea position in Fig. 6, the solenoid operated valve 88 and fuel pump motor 89 are immediately energized. When this occurs fuel from vessel 41 can flow through conduit 48 to fuel pump 46 from which the fuel is discharged through conduits 50 and 44 as far as the solenoid operated valve 10 which is still deenergized and hence in its closed position. Simultaneously with the energization of fuel pump motor 89, the ignitor 24 is energized and becomes heated to a high temperature.

After a definite interval of time, such as thirty seconds, for example, the time delay relay 8l becomes effective to close contacts 83, 84 and 86, 81, whereby the solenoid operated valves t9v and 19 and electric motor 48 are simultaneously energized. The electric motor All is then rendered operable to drive the air circulating fan 21 and exhaust and combustion air impellers 38 and 39 of the blower. The impeller 39 causes combustion air to be drawn through the altitude compensator- 54, manifold 55 and conduit 43 into the lower impeller chamber 3B, as previously described. The energization of solenoid operated valves 69 and 18 causes opening of both of these valves, so that fuel may be delivered from the fuel pump 46 to the spray nozzle 45 through pressure regulator valve 51 and solenoid operated valve B9 in conduit 58 and through solenoid operated valv in conduit 44. i

Since both solenoid operatedr valves 69 and 'ID open when switch 12 is in the High heat position and pressure regulator valve 6l is adjusted-to permit fuel to be supplied at a higher fixed delivery pressure than pressure regulator valve 68., the adjustment of pressure regulator valve 6l determinesv the rate of flow of fuel to the spray nozzle 45. The fuel sprayed upwardly into. theA impeller chamber 36 from spray nozzle 45 strikes the rotating impeller 39 and' is mechanically atomized and mixes with combustion aire, andi the mixture passes through passage 45, into the burner inlet chamber 2l. From inlet chamber 2l the combustible fuel mixture flows through the passages 22 into the lower part 250i heating space 1.8 in which ignition of the mixture is` accomplished by the ignitor 24.

After ignition has been accomplished in the lower part 25 of heating space I6 and the latter reaches a predetermined temperature, .such as' 180 F., for example, the normally closed thermal switch 90 opens. With opening of thermal switch 9D, a high resistance circuit is completed for ignitor 24 from contact D through the electric lamp 9| which then becomes lighted on panel 1,8

to indicate that combustion is taking place in;

heater 9 and heating of air circulated by fan 2'! is being effected. By connecting the high resistance electric lamp 9i in series with ignitor 24, only a small current will pass through the ignitor during normal operation of the heater and thus prevent overheating of the ignitor which is constantly subjected to the high temperatures developed in the lower part of the heating space The thermal switch 85 is normally closed and opens when undesirable overheating of the heater occurs. When overheating of the heater does take place, and the air is heated to a predetermined high temperature of about 450 to 500 F., for example, the thermal switch 85 opens and the circuit for the fuel pump motor 89 and solenoid operated valve 88 is opened. When this occurs solenoid operated valve 88 closes and the fuel pump 46 becomes ineffective to supply fuel to spray nozzle 45. However, opening of the thermal switch 85 does not affect the energization of' heating coil 88 of time delay relay 8l and motor 4D, so that the latter will continue to drive the air circulating fan 21 and the exhaust and combustion air impellers 38 and 39, respectively. The continued operation of fan 21 when overheating occurs tends to bring about rapid cooling of the heater. When the temperature of the heater falls below the predetermined high temperature, the thermal switch 85 again closes to energize solenoid operated valve 88 and fuel pump motor 89 to permit fuel to be pumped to spray nozzle 45.

Thus, when overheating of the heater occurs the heater does not shut down completely but intermittent operation takes place which is extremely desirable in airplane heating. In the event the operating condition causing overheating persists, the heater will operate intermittently to effect heating of air until overheating occurs at which times combustion of the air and fuel mixture in the lower part of heating space I5 temporarily stops. When the air temperature falls suiiciently to effect closing of thermal switch 85, operation of the heater 9 is again resumed in the manner just explained.

With rotary switch 12 in the High heat position the heater 9 is operated at a high heat output and the series motor operates in one speed range with the motor speed increasing with increase in altitude of the airplane. During such operation of heater 9, fuel is supplied to the heater both at sea level and high altitude operation at a substantially fixed delivery pressure dependent upon the adjustment of the pressure regulator valve 51 which is adjusted to deliver fuel to spray nozzle at a higher fixed delivery pressure than pressure regulator valve 68.

When it is desired to operate heater 9 at a low heat output the control member 1| is turned to the Low position on panel 18, so that rotary switch l2 will be movedv to the Low heat position in Fig. 5. In such position of rotary switch 12 the switch arms 16 and 11 engage contacts C and D and circuits are completed for the fuel pump motor 89, solenoid operated valve 89 and ignitor 24 from contact D, in the same manner explained above when the rotary switch 12 is in the High heat position in Fig. 6. However, since contact E is not engaged by any switch arm in the Low heat" position in Fig. 5, solenoid operated valve 69 remains deenergized and in its closed position when the heater is being operated at low heat output.

In the Low heat position of rotary switch 12 a circuit is completed from contact C for heating coil of time delay relay 8|, After a definite interval of time circuits are completed for the series motor 130 and solenoid operated valve 'I0 by the closing of contacts 83, 84 and 8d, Si of relay 8i, in a manner similar to that explained above in connection with the operation of heater 9 at high heat output. However, a resistor l0 is now connected in series with the electric motor which will operate in a lower speed range than during operation of the heater at high heat output. Further, since only solenoid operated valve 'Iii is energized to cause the-latter to open to its open position, fuel can only flow from fuel pump Mi to spray nozzle through conduit 50, and pressure regulator valve 6B and solenoid operated valve 'i0 in conduit lill. Since pressure regulator valve 08 is adjusted to deliver fuel to spray nozzle i5 at a lower fixed delivery pressure than the pressure regulator valve 01, fuel will be supplied to the spray nozzle i.

at a lower rate than when operating heater 9 at a high heat output.

Under certain conditions it may be desirable to operate heater 0 to provide ventilation without effecting heating of .the interior of an airplane. lThis is accomplished by moving the control member 'il to the Vent position on panel "I3 so that rotary switch 'i2 will be moved to the Vent position in Fig. l. In this position of rotary switch i2 switch arm 'i5 engages contact B and a single circuit is completed from this contact for energizing motor (lil. Under these conditions air is simply circulated through the heater 9 by fan 2l to provide the desired ventilation within the interior of an airplane with no heating of air being effected while flowing through the annular spaces I0 and i0.

In a heater having a control system for operating the heater at high and low heat outputs in a manner similar to that just described, and which is provided with an altitude compensator like that illustrated in Fig. 1, the fuel pump lit is capable of delivering fuel at a pressure of about l0 lbs. per sq. in., and the pressure regulator valves 58 and i are adjusted to deliver fuel at pressures of 2 and 5X1. and 5 lbs. per sq. in., respectively, to a precision spray nozzle having a discharge orifice of .010 inch in diameter.

The motor driving the air circulating fan and exhaust and combustion air impellers, and which corresponds to motor 10, is a high speed series motor rated at .46 H. P. and operable from a 28 volt source of electrical supply. The motor speed at sea level is about 11,000 R. P. M. for operation at high heat output and the motor speed increases about 1,000 R. P. M. per 10,000 feet of altitude. The resistor in the motor circuit, which corresponds to resistor 'i9 in Fig. 2, is effective to reduce the motor speed to about 6,000 R. P. M. for sea level operation of the heater at low heat output.

When the heater is operated at high heat output and high octane gasoline is being supplied to the spray nozzle at a fixed delivery pressure of about 5 lbs. per sq. in., the fuel is delivered at a rate of about 3 lbs. (1/2 gallon) per hour; and when operated at low heat output and gasoline is being supplied at a xed delivery pressure of 2 and 1%, lbs. per sq. in., the fuel is delivered at approximately one-half this rate. The heater at sea level under these conditions develops about 45,000 B. t. u. per hour at high heat output and approximately 25,000 B. t. u. per hour at low heat output.

The heater at both low and high heat outputs l0 is capable of producing an average temperature rise of lil. when air to be heated is circulated through the heater, and discharges heated air at the rate of about 225 cubic feet per minute at low heat output and about 450 cubic feet per .minute at high heat output.

The optimum weight ratio of air to gasoline is about 13 to 1 which is maintained satisfactorily by the altitude compensator 5 for both low and high heat output operation of the heater. Although the heater may be operated on the ground over a wide range of weight ratios of air to fuel, the 13 to 1 ratio referred to above is for sea level operation as well as at high altitudes because this ratio is necessary for stable and reliable operation =at high altias explained above. rIhe altitude compensator 5d is adjusted so that compensation for decrease in air density begins at an altitude of about 15,000 feet at which time the bellows d@ is sufficiently extended for combustion. air to start passing into manifold 55 through the slots in the side wall of valve member 03.

1t will now be understood that an improved altitude compensator has been provided for an airplane heater which is capable or" maintaining a desired weight ratio of air to fuel when an electrical translating device having the operating characteristics of a series wound motor is employ-ed to drive the fan or impeller of the blower arranged to supply air for combustion in the heater. Moreover, the altitude compensator is especially useful for an airplane heater arranged to be operated at low and high heat outputs by operating an electric motor in two speed ranges in each of which liquid fuel is supplied at a different fixed delivery pressure, and in which such motor drives the fan for circulating air to be heated and an impeller of a blower which supplies air for combustionl in the heater. Further, a control system has been provided which i-s relatively light in weight and reliable in operation so that an airplane heater may be operated at low and high heat outputs or simply provide ventilation by turning or actuating a single control member.

Although a single embodiment of the invention has been shown and described, it will be apparent to those skilled in the art that changes can be readily made and that certain features can be used independently of others without departing from the spirit and scope of the invention, as pointed out in the following claims.

What is claimed is:

1. In a heating system including a heater, means to supply a combustible mixture of air and fuel to the heater including a blower and a fuel supply line, means including a fan for circulating air to be heated past the heater, two pressure regulator devices connected in the fuel supply line to deliver fuel only at two fixed pressures to the heater, electrically operated valve means connected in the fuel line to permit fuel to be supplied to the heater at one or the other' higher xed delivery pressure, an ignitor for igniting the combustible mixture supplied to the heater, an electric motor for driving the blower and ian, two circuits for the motor to operate the latter only at two speeds or speed ranges, a time delay relay having an element movable from a rst position to a second position after an interval of time following initial energization, switch means movable between an off position and two heat operating positions, means includ` ing the switch means in each of its heat operating positions to energize the ignitor and the time delay relay, means including the switch means in one heat operating position and the time de,- lay relay when the element has moved to its second position to energize the motor through one of the circuits associated therewith and the electrically operated valve means so that fuel will be supplied to the heater at one fixed delivery pressure and the motor will operate at one speed or speed range, and means including the switch means in the other heat operating position and the time delay relay when the element has moved to its second position to energize the motor through another of the circuits associated therewith and the electrically operated valve means so that fuel will be supplied to the heater at the other higher fixed delivery pressure and the motor will operate at the other higher speed or speed range.

2. An aircraft heater comprising structure providing a heating space adapted to be heated by combustion of a mixture of air and fuel, means to supply fuel for said combustion only at two fixed delivery pressures including electromagnetically operable valve means and two pressure regulators, means including a blower and a conduit connected thereto for supplying combus tion supporting air for said combustion, a series motor for driving said blower, means for operating said motor only in tWo xed speed ranges, circuit means including a single control member for simultaneously controlling the fuel supply 12 means and the motor operating means so that fuel will be supplied for said combustion at one fixed delivery pressure through a portion of said valve means and one pressure regulator when the motor is operating in one xed speed range and through said valve means and the other pressure regulator at the other higher fixed delivery pressure when the motor is operating at the other higher fixed speed range, and valve means associated with the conduit operable responsive to atmospheric pressure for controlling flow of combustion air supplied for said combustion by said blower.

CLARENCE H. BENNETT.

REFERENCES CITED The following referenlces are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,179,846 Finnigan Nov. 14, 1939 2,223,283 Grant et al Nov. 26, 1940 2,264,869 Beardsley Dec. 2, 1941 2,308,555 Tate Jan. 19, 1943 2,313,149 Jacobsson Mar. 9, 1943 2,314,089 Hess et al Mar. 15, 1943 2,321,940 Robertson June 15, 1943 2,353,921 Myler July 18, 1944 2,364,214 Hess et al Dec. 5, 1944 2,400,116 Holthouse May 14, 1946 2,401,393 Wakefield June 4, 1946 

